Thin film semiconductor devices such as thin film transistors (TFTs) have been used in active matrix display devices for liquid crystal displays, or solid-state imaging devices for digital cameras. In displays, TFTs are used as switching elements which select pixels, driving transistors which drive pixels, or drivers disposed in regions out of display regions.
For example, organic EL display devices including organic electroluminescent (EL) elements including organic luminescent materials are current-driving display devices rather than voltage-driving liquid crystal displays. For this reason, an urgent development of TFTs having higher performance is required. Recently, TFTs including oxide semiconductors, such as InGaZnO semiconductors, have been actively developed as channel layers (see Patent Literature 1, for example).